Certain pieces of radio equipment, such as certain radio conversion receivers and/or transmitters, are provided with balanced circuit devices, such as mixers or amplifier circuits.
Due to their layouts the balanced circuit devices, and notably those used in direct or low-intermediate frequency (IF) conversion receivers, present dissymmetry and imbalance. As it is known by one skilled in the art, the presence of such dissymmetry and imbalance increases even-order non-linearities. Due to these even-order non-linearities, a higher even-order intermodulation appears: two strong interferers that are close to the frequency channel of interest, generate a low-frequency distortion. With this low-frequency distortion appearing in the low-frequency band where the demodulated wanted signal stands, it is difficult to separate the wanted signal from the (unwanted) distortion.
For rejecting this high even-order intermodulation, the balanced circuit devices must be coupled to a dedicated device allowing the achievement of a high second-order input intercept point (or IIP2).
An example of such a dedicated device is notably described in the patent document WO 02/084859. More precisely, this patent document describes the use of one or two “IIP2 calibration devices” that aim at adjusting first and second load impedance means, respectively coupled to first and second outputs of a balanced circuit device, to a load imbalance for linearization purpose. Each (IIP2) calibration device comprises switched resistors connected in parallel and tuned so that a proper imbalance of the first and second load impedance means balances the balanced circuit device.
The imbalance introduced in the balanced circuit device is small, so that it has a negligible effect on its other performance parameters such as the gain, the noise or the third order input intercept point (or IIP3). But, the dissymmetry and imbalance of the balanced circuit device being generally very small, switched resistors of high value need to be connected in parallel to compensate the introduced imbalance. So when each balanced circuit device output is provided with its own (IIP2) calibration device, the area occupied by these two (IIP2) calibration devices is too large. And when only one of the two balanced circuit device outputs is provided with a (IIP2) calibration device, its dissymmetry and imbalance cannot be properly compensated. More precisely, this allows only to compensate the dissymmetry in a single way, because adding resistor(s) to a load always lowers this load.
So, the object of this invention is to improve the situation.